The genes coding for the rRNAs seem evolutionary conserved on the first glance, but astonish one with their variability in the structure and a variety of functions on closer examination. The non-coding parts of rDNA contain regulatory elements, protein binding sites, pseudogenes, repetitive sequences, and microRNA genes. Ribosomal intergenic spacers are not only in charge with the nucleolus morphology and functioning, namely, the rRNA expression and ribosome biogenesis, but also control nuclear chromatin formation thus mediating cell differentiation. The alterations in the expression of these non-coding regions of rDNA in response to environmental stimuli underlie the keen sense of a cell to various types of stressors. Malfunctioning of this process may result in a wide range of pathologies from oncology to neurodegenerative disease and mental illness. Here, we observe to-date materials on the structure and transcription of the ribosomal intergenic spacer in humans and its role in rRNA expression, in-born disease development, and cancer.